1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
In the field of wireless data systems, a number of well-known standards, such as 1x-EV-DO, 1xEV-DV as well as the High Speed Downlink Packet Access (HSDPA) specification in the Universal Mobile Telecommunication System (UMTS) standard, have been employed. Newer technologies such as fast scheduling, adaptive modulation and coding (AMC) and hybrid ARQ (HARQ) have also been introduced to improve overall system capacity. However, application of the above-mentioned techniques has been limited to transmitting data blocks for unicast traffic, i.e. a data block addressed to a single mobile station. In general, a scheduler selects a user for transmission at a given time and adaptive modulation and coding is used to select an appropriate transport format (modulation and coding) for current channel conditions seen by the user. Due to errors in channel quality estimates, a relatively high level of frame errors may occur in the transmissions performed at a given rate (transport format). Hybrid ARQ has been employed to recover from transmission errors without significant loss in throughput.
An example of hybrid ARQ operation for the 1xEV-DO system is shown in FIG. 1. The hybrid ARQ transmissions use a 4-slot interlacing structure, i.e. the hybrid ARQ retransmissions for an original transmission in slot n happens in slots (n+4), (n+8), and so on. A total of 4 interlaces are available for transmission to a single user or for transmissions to different users. In the example shown in FIG. 1, a new or first data transmission occurs in slot 2 on interlace 2. In the exemplary scenario illustrated in FIG. 1, the transmission is unsuccessfully received and the receiver sends back a negative acknowledgement signal (NACK). The NACK indicates to the transmitter that the transmission was not properly received, causing the transmitter to retransmit the same data in slot 6 (again on interlace 2). The receiver combines the retransmitted data with the previously received first transmission, and based on the two pieces of data, the transmission is successfully decoded. Those skilled in the art will appreciate that the process of retransmitting and combining may be repeated until the data is successfully received (early termination, as indicated by an ACK) or a fixed number of attempts have been made. Once the data is properly received, the receiver sends back an acknowledgement signal (ACK). The transmitter then starts another new transmission on interlace 2 in slot 10. Similarly, the transmissions happens in parallel on other interlaces, such as 1, 3 and 4.
Unlike unicast traffic, broadcast/multicast data blocks are addressed to more than one receiver or mobile station. In a broadcast transmission, the data blocks are addressed to all the mobiles in the system, whereas in a multicast transmission, the data blocks are addressed to a subset of mobiles in the system. In general, no feedback is required from the mobile stations. Generally, in both multicast and broadcast transmissions, the data blocks are transmitted on a predetermined number of slots, i.e. there is no early termination due to hybrid ARQ ACK feedback.
A stylized representation of a wireless system capable of broadcast data packet transmission is shown in FIG. 2. The broadcast data packet contains information from one or more broadcast streams carrying broadcast programs. In general, two layers of channel coding are used to provide robustness against errors. The first layer of coding also called outer code is performed using well-known Reed-Solomon code. The Reed-Solomon code adds some redundancy to the data. The Reed-Solomon coded block is then segmented into smaller data blocks for Turbo coding. A number of subpackets (e.g., SP1-SP3) from the same data block are created at the output of the Turbo coding. In general, the data block can be recovered from any one of the received subpackets (SP1-SP3) as long as the coding rate is smaller than 1. Table 1 shows data rates for a 3072 bit data block transmitted within one, two or three slots (subpackets). A subpacket is transmitted within a slot of duration 1.67 ms. The received subpackets at the mobile receiver are used to recover the data block. The data blocks are then reassembled to form the broadcast packet.
TABLE INumber of subpackets (slots)transmissionData Rate11843.2Kb/s2921.6Kb/s3614.4Kb/s
The broadcast and unicast traffic in the 1xEV-DO system is multiplexed on an interlace-by-interlace basis. In the example shown in FIG. 3, interlace 1 is used for broadcast traffic. The broadcast data block is transmitted in three subpackets (SP1, SP2 and SP3) on three slots i.e. slot # 1, 5 and 9 from the entire system, i.e., all of the base stations in the system. Therefore, the mobile station can potentially receive and combine signals from multiple base stations. The mobile station also combines SP1, SP2 and SP3 transmissions in order to recover the broadcast data block. The SP2 and SP3 transmissions contain additional redundancy for broadcast data block recovery.
The interlace-based multiplexing approach used in the prior art poses problems when different broadcast data rates are used by different base stations in different cells in the system. The use of different data rates in different cells may be the case in a system deployment where the cell sizes are different. This, for example, can be the case, for a downtown area surrounded by suburbs and rural areas. The cell size in densely populated areas is smaller in order to provide more cell sites to accommodate the larger amounts of traffic. However, as the population density decreases in the surrounding suburbs and rural areas, the effective cell sizes increases. The smaller cells deployments can in general support higher data rates because of the smaller path loss due to relatively shorter distance between the base station and the mobile station. The larger cells have, in general, larger path loss and therefore cannot support very high data rates. An example of cell layout showing three sets of cells is stylistically shown in FIG. 4. A set of 7 center cells is labeled as set A. A first and second ring of cells around set A are labeled as set B and set C, respectively.
An example of broadcast transmissions at different data rates in different sets of cells is shown in FIG. 5. In this example, set A transmits only SP1 of the broadcast data block, achieving the highest transmission data rate. Set B transmits both SP1 and SP2 of the broadcast data block therefore achieving half the data rate of set A rate. Similarly, set C achieves one-third rate of set A because the broadcast data block is transmitted in three subpackets. Note that SP2 and SP3 contain additional redundancy. Therefore, if a transmission can be decoded using a smaller number of subpackets, the achieved information data rate is higher.
An example of a broadcast transmission over three interlaces is shown in FIG. 6. Each of the interlaces carries a broadcast data block consisting of one, two or three subpackets. In this example, sets A, B and C transmit the broadcast data block in one, two and three subpackets, respectively. The fourth interlace is used for the unicast traffic. In set C, slots 1, 2, 3, 5, 6, 7, 9, 10, 11 are used for the broadcast traffic while slots 4, 8 and 12 in interlace #4 are used for the unicast traffic. In FIG. 6, SPij denoted the jth subpacket from the ith data block. For example, SP21 represents the first subpacket from the second data block. The subpackets transmitted from multiple cells at the same time with the same subpacket number can potentially be soft combined at the receiver to assist in decoding the data packet. In FIG. 6, SP11, SP21 and SP31 are transmitted from all the three sets of cells A-C at the same time, and, therefore, these subpackets received from all the cells are combined at the receiver. Similarly, SP12, SP22, and SP32 are transmitted from cell set B and cell set C. Therefore, these subpackets are soft combined from cell set B and cell set C. On the other hand cell set A may potentially be transmitting unicast traffic during slots 5, 6 and 7 when SP12, SP22 and SP32 are transmitted from cell set B and cell set C. Therefore, transmissions from cell set A potentially interfere with transmissions from cell set B and cell set C. SP13, SP23, and SP33 are transmitted from cell set C only. Therefore, these subpackets potentially get interference from both cell set A and cell set B.
In cell set B, slots 9, 10 and 11 are not used for broadcast traffic because the broadcast data blocks are transmitted in two subpackets only. Therefore, these free slots can potentially be considered for transmission of other information, such as unicast traffic. However, the unicast traffic uses hybrid ARQ and potentially requires multiple retransmission attempts. For example, if a unicast data block transmission is started in slot#9, the retransmission needs to happen in slot#13, but slot#13 belonging to interlace#1 is reserved for a broadcast data block transmission. Therefore, a retransmission cannot be performed for unicast traffic. Similarly, in cell set A, slots 5, 6, 7, 9, 10 and 11 become available, but like slots 9, 10 and 11 in cell set B, these slots may not be used for unicast traffic due to restrictions on retransmissions. Thus, these unused slots remain unavailable, and, therefore, the multiplexing approach used in the prior art poses serious restrictions on scheduling and results in system inefficiency.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.